Create and publish support content for CSV downloads

[h-m-f-t]

Issue description

In cisagov/manage.get.gov#2323, we're allowing users to download an export of their domain information. Though most of the fields in this CSV will be self-explanatory, we should write and publish some support content on get.gov (probably https://get.gov/help/domain-management/) so users have a key for the headers. That way, they can self-service and don't have email to ask questions about it.

Acceptance criteria

• Draft content to help users know what's in the domain CSV
• Publish it on get.gov

Additional context

• It might be nice to just include the key with the CSV, but you can't make multi-sheet CSVs.
• We need to make sure we link to this new content from inside the registrar so users can find it. (Maybe add this to #2323?)

Links to other issues

• 🔄 Relates to Domain managers can export their own domain data to a CSV file manage.get.gov#2323

[h-m-f-t]

I've added this to the sprint 50 (along with cisagov/manage.get.gov#2323) but that may not be desired.